A fiber-optic gyroscope measures an angular velocity of a fiber coil by utilizing the fact that the angular velocity is in proportion to the phase difference between a clockwise-propagating beam and a counterclockwise-propagating beam (Sagnac Effect). Optical fiber gyroscopes can be classified by the methods of modulation of signals, e.g. phase modulation type, frequency modulation type, or phase-shift type. For example, the phase modulation type modulates the phase of light beams propagating in a fiber coil by expanding or shrinking a part of the optical fiber in the vicinity of the fiber coil at a definite frequency (modulation frequency). A photodetector detects the intensity of interfering (clockwise and counterclockwise) beams. The output of the photodetector includes a fundamental wave of the modulation frequency and the harmonics as an expansion of the coefficients of Bessel functions. Thus, the fundamental signal or arbitrary harmonics can be obtained by synchronous demodulation of the output signal by a modulation frequency carrier or harmonics carrier which is yielded from the modulation signal.
Since a fiber-optic gyroscope makes a clockwise beam interfere with a counterclockwise beam, the polarization planes of both beams must be the same. Coincidence of the polarization planes enables two beams to interfere with each other. When the polarization planes of the beams are different, the intensity of the interfering beams reduces by a factor of cosine of the angle held between two polarization planes. If the polarization planes are perpendicular to each other, two beams cannot interfere at all.
A fiber coil is produced from a single-mode fiber in ordinary cases. A single-mode optical fiber allows the rotation of polarization planes, because two beams with polarization planes perpendicular to each other degenerate with regard to the phase constants (propagation constants). If the polarization planes accidentally rotate in a single-mode fiber, the polarization planes of clockwise- and counterclockwise- beams become different and the intensity of the output power of the photodetector will fluctuate according to the rotation of the polarization planes.
K. Boehm et al. had proposed an improved fiber-optic gyroscope provided with a depolarizer in a part of a fiber coil to avoid the fluctuation of the output signal owing to the rotation of the polarization planes.
K. Boehm et al.: "Low-Drift Fiber Gyro Using a Superluminescent Diode", ELECTRONICS LETTERS, Vol. 17, No. 10, p. 352(1981)
The gyroscope has used a Lyot Depolarizer which is constructed with two birefringent materials with thicknesses in a ratio of 2:1, being bonded with optical principal axes twisted at 45 degrees. A depolarizer depolarizes any light beams, e.g. linearly-polarized beams, circularly-polarized beams or elliptically-polarized beams. A depolarizer enables clockwise- and counterclockwise-propagating beams to interfere with each other, irrespective of original states of polarization before the beams pass through the depolarizer. Accidental rotations of the polarization planes in a single-mode fiber coil had been solved by supplying a depolarizer to a part of the fiber coil. Even if the polarization of beams rotated by 90 degrees in the fiber coil by some reasons, half of the power of beams can pass through the polarizer in the reciprocal direction because the beams are fully depolarized and can interfere at the photodetector. Without a depolarizer, the beam with a 90 degree rotated polarization plane could not pass through the polarizer at all in the reciprocal direction.
However, Boehmps fiber-optic gyroscope with a single depolarizer had a new drawback that the adjustment of the polarization of the original beam just emitted from a light source with the polarization plane of the propagating beams became impossible, because the newly-installed depolarizer vanishes the memory of the polarization of the propagating beams. Without a depolarizer, it is possible to maximize the output power at the photodetector by harmonizing the direction of the polarization of the initial beam just emitted from the light source with the allowable axis of the polarizer. On the other hand, Boehm's gyroscope have not solved another inherent difficulty of polarization rotation in the single-mode fiber between the light source and the polarizer.
To solve the difficulties more completely, the Inventors have proposed a more advanced fiber-optic gyroscope provided with two depolarizers, i.e. first depolarizer in front of the polarizer and second depolarizer in the vicinity of the fiber coil like Boehm's fiber-optic gyroscope. The first one was a novel point in the gyroscope. It has been disclosed by;
(1) Japanese Patent Laying Open No. 4-106416 (106416/1992) PA0 (2) Japanese Patent Laying Open No. 4-106420 (106420/1992) PA0 (3) Japanese Patent Laying Open No. 4-106417 (106417/1992) PA0 (4) Japanese Patent Application No. 3-198534 (filed on Jul. 12, 1991) PA0 (5) Japanese Patent Application No. 4-139899 (filed on Apr. 30, 1992) PA0 (4) has proposed a quasi-depolarizer constructed with a polarization maintaining fiber spliced to a front end of a fiber-type polarizer with their optical axes twisting at 45 degrees with each other. A fiber-type polarizer is a fiber coil made from a polarization maintaining fiber. This is one of polarizers. A polarizer has an allowable axis which is defined as the direction of polarization of the beams passing through the polarizer without loss. It has also a forbidden axis which is perpendicular to the allowable axis. The beam having the polarization plane parallel with the forbidden axis cannot pass through the polarizer. If the optical principal axis inclines at 45 degrees to the allowable axis of the polarizer, an ordinary beam and an extraordinary beam transmitted in the birefringent material are exactly divided in half into partial beams with the polarization planes parallel with the allowable axis and the other partial beams with the polarization planes parallel with the forbidden axis of the polarizer. The latter partial beams vanish in the polarizer. Only the former partial beams can pass through the polarizer. Thus, the ordinary beam and the extraordinary beam appear at the rear end of the polarizer at different states of phase. Of curse, the product of the length L of the polarization maintaining fiber and the birefringence B=n.sub.x -n.sub.y has been determined to be longer than the coherent length C of the light source. Namely, BL&gt;C. This inequality must hold in a depolarizer in every case.
Among three, (1) used a normal type depolarizer having two polarization maintaining fibers spliced together with their optical axes twisted at 45 degrees. The ratio of lengths of the polarization maintaining fibers was 2:1. This was a kind of Lyot depolarizer constructed with the polarization maintaining fibers. The length of the shorter one was determined by the condition that the optical path difference due to birefringence should be longer than the coherent length of the light emitted from the light source. Such a depolarizer has well-known as mentioned before as a Lyot depolarizer. Birefringent crystals were only replaced by birefringent optical fibers (i.e. polarization maintaining fibers). FIG. 7 exhibits a schematic view of the depolarizer constructed with fibers. FIG. 6 is a schematic view of an optic fiber gyroscope with two depolarizers proposed by the Inventors for the first time.
(2) constructed a substantial depolarizer consisting of a single polarization maintaining fiber facing toward a light source generating linearly-polarized light beams, in which an optical axis of the polarization maintaining fiber is twisted at 45 degrees to the polarization direction of the light source.
(3) constructed a substantial depolarizer consisting a single birefringent crystal placed in a special optical path of the beams just emitted from a light source with an optical principal axis twisted at 45 degrees to the polarization direction of the emitted linearly-polarized beams. A substantial depolarizer was produced by the polarization of the light source and a birefringent crystal.
All the three have proposed gyroscopes having two depolarizers. (1) has two independent, genuine depolarizers. (2) and (3) have a genuine depolarizer and a substantial depolarizer taking advantage of the polarization of the emitted light beams. Then, the Inventors have discovered the fact that one birefringent material becomes dispensable when a depolarizer shall installed in a fiber-optic gyroscope, adjoining a polarizer. Namely, only a single birefringent material can construct a substantial depolarizer adjacent to a polarizer. This depolarizer is an incomplete depolarizer dependent upon a polarizer. But the function is fully the same as an independent depolarizer. This is a depolarizer in a broad sense. Therefore, it will be called a quasi-depolarizer from now on. The Inventors have disclosed such a quasi-depolarizer by;
FIG. 8 demonstrates the fiber-optic gyroscope proposed by the applications (4) and (5). There are two depolarizers. A first depolarizer just in front of a polarizer is a quasi-depolarizer. A second depolarizer lying in the vicinity of a fiber coil is a complete, independent depolarizer.
(5) used a general polarizer instead of a special type of depolarizer. Besides the first depolarizer, the second depolarizer was also simplified (mono-birefringent material) at the rear end of the polarizer. (5) was a generalization of (4). In any cases, the fiber-optic gyroscope required two depolarizers.
The Inventors believe that another depolarizer shall be provided between a light source and a polarizer, if there is a single-mode fiber between the light source and the polarized to avoid the fluctuation of the output signal owing to the rotation of the polarization planes. Inventions (1) to (3) belonged to such improvements requiring two depolarizers. Inventions (4) and (5) simplified the structure of a depolarizer.
Nobody has proposed the same improvements of fiber-optic gyroscope except the Inventors. Therefore, at present, nobody take notice of difficulties or problems regarding a new fiber-optic gyroscope having two depolarizers except the Inventors.
A depolarize is an optical device for converging a linearly-depolarized, circularly-polarized or elliptically-polarized beam into a depolarized beam. Essentially, two birefringent materials with thicknesses in a ratio of 2:1 are coupled with each other at 45 degrees of an angle between the optical axes of the materials. In the case of polarization maintaining fibers, the fundamental structure is the same. Furthermore, the difference of optical paths due to birefringence must be longer than the coherent length of the light emitted from a light source. Two differently-polarized (ordinary and extraordinary) beams cannot interfere, since the optical path difference is longer than the coherency of the light. Since the ratio of lengths is 2:1, partial beams with the same polarization plane are spaced by a common distance longer than the coherent length. Such partial beams do not interfere because of the distance longer than the coherent length, although they have the same polarization plane. A cross-term in the square of the amplitude vanishes. Thus, the square of the amplitude becomes constant for all partial beams with different polarization planes. All partial beams with different polarization planes have the same energy. This is a depolarized state. The long optical path difference eliminates interference of the beams with the same polarization planes. A single depolarizer only requires such simple conditions (BL&gt;C, ratio=2:1).
However, some difficulties will occur when two depolarizers are included in optical paths of a fiber-optic gyroscope. Will no interference between two beams which have once an optical path difference longer than the coherent length by passing through a depolarizer occur forever? Can the function for separating the optical paths of a depolarizer ensure everlasting non-interference between the once separated beams? The two beams can interfere with each other. When two beams pass through the other depolarizer, the birefringence of the second depolarizer will affect against the once-obtained separation between two beams. Sometimes the birefringence will decreases or cancel the path difference which has been endowed by the first depolarizer. In some cases, the optical path difference will become shorter than the coherent length of the light. Then, two beams will interfere! Rotation of polarization planes often occurs in a single-mode fiber, because phase constants of the beams degenerate in a single-mode fiber. If the rotation of the polarization planes occurred, the intensity of the interference beams would fluctuate, because the two once-separated beams with the same polarization planes would interfere. The fluctuation of the interference beams appears as a noise, a drift or change of the scale factors of the signals.
Such a problem would appear in an gyroscope for the first time, when the Inventors introduce two depolarizers into it. Thus, the Inventors have noticed the difficulties in a twin-depolarizer gyroscope for the first time. Nobody take notice of such a problem. A purpose of this invention is to solve the problem of a twin-depolarizer gyroscope.